The present invention relates to mixtures of siloxanes and to the use thereof as heat transfer fluid.
Organosiloxanes and organopolysiloxanes (silicone oils), referred to collectively hereinafter as “siloxanes” for short, frequently find use as heat transfer fluids because of their high thermal stability and wide liquid range and the low degree to which their viscosity depends on temperature. DE 2754705 A1 details the advantages of siloxanes over other heat transfer agents. Especially within the range of very low (below −50° C.) or very high temperatures (200-400° C.), they are superior to organic heat transfer fluids or are the only nonionic heat transfer fluid that is usable at all. For example, EP 1473346 B1 describes mixtures of linear and cyclic dimethylpolysiloxanes usable as coolants down to −100° C. In addition, the brochure “SYLTHERM 800 Heat Transfer Fluid—Product Technical Data” from The Dow Chemical Company (CH 153-046-E-1097, October 1997) describes a linear, permethylated silicone oil (“Syltherm 800”) and states the maximum prolonged use temperature as 400° C. (closed system with exclusion of air). It is also stated there that, in the case of brief thermal stress, up to 538° C. is attainable without substantial breakdown.
These properties of the siloxanes make them ideal for use as high-temperature heat transfer fluids, for example in solar thermal power plants, especially in those with parabolic trough and Fresnel technology, where the heat transfer fluid is subjected to high thermal stress up to 400° C. and severe temperature variations for a number of years. The use of silicone oils in solar thermal devices is described in DE 2754705 A1, U.S. Pat. No. 4,122,109 and U.S. Pat. No. 4,193,885.
The composition of siloxane mixtures is temperature-dependent by virtue of rearrangement processes, and as a result is also time-dependent until attainment of the equilibrium state at the chosen temperature. In the case of linear, permethylated silicone oils, the attainment of the equilibrium state at 400° C., for example, can take a few days. In parallel, there is thus also a change in the physical properties. The result of this can be that important operating parameters of a device operated with a siloxane mixture as heat transfer fluid, for example the vapor pressure or viscosity, change considerably with time. This is disadvantageous, since it can necessitate additional expenditure on control and regulation or even additional expenditure in the construction of the device, or the device may be usable only to a limited degree, if at all, within this period.
Patent specifications U.S. Pat. No. 4,122,109 and U.S. Pat. No. 4,193,885 describe the addition of metallic stabilizers and optionally of hydrogen-containing silicon compounds to noncyclic methylpolysiloxanes, in order to suppress the temperature-dependent change in the chemical composition and hence to keep the composition and the physical properties stable over time. However, it is clear from the examples that the rearrangements cannot be entirely suppressed. The abovementioned accompanying product brochure (“Syltherm 800”) states that the rearrangement processes are slowed significantly but do take place, and the equilibrium state is ultimately attained after a few months. This is associated with a considerable rise in the vapor pressure. However, since heat transfer fluids, for reasons of cost, are used in solar thermal power plants for a number of years, addition of stabilizers is therefore unsuitable for this application, since it cannot in fact prevent the rearrangements over this period, and is actually disadvantageous as a result of the increase in material costs that it causes.
It is therefore an object of the invention to provide siloxanes which, after attainment of a particular temperature, have nearly constant physical properties over time, in order to avoid the disadvantages mentioned.